Abstract: In order to improve the surface hardness of 40CrNiMoA steel and improve its surface wear resistance, the surface of 40Crnimoa steel was heat treated by high frequency quenching. The results show that: After high frequency quenching heat treatment in water, oil and salt solution, the microstructure of 40CrNiMoA steel is mainly residual austenite, tempered martensite and granular carbide. When the cooling medium is salt solution, the tempered martensite microstructure is finer and more uniform, and the sample has higher surface hardness, yield strength, tensile strength and wear resistance, but its plasticity is reduced. The fracture form of oil and salt quenched specimens is ductile fracture, while the fracture form of water quenched specimens is quasi-cleavage fracture.